BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 26

Oscilloscope

Biopotential Amplifier

10mV p-p

+I N

10Hz

OUT

Input 1 Input 2

-IN

(a)

Oscilloscope

Biopotential Amplifier

10mV p-p

10Hz

-IN

OUT

Input1 Input2

+IN

(b)
Figure 1.23 The gain of a differential biopotential amplifier can be measured by injecting a signal (e.g., 10-Hz sinusoidal) at an amplitude
similar to that expected from the biopotential (e.g., 1 mV) to the inputs. ( a ) When the amplifer is configured as single-ended, the output sig-
nal should be an amplified version of the input signal. ( b ) When the circuit is reconfigured to use the inverting input, the output signal should
be an amplified and inverted (opposite phase) version of the input signal.

4. Calculate the common-mode gain G CM V out CM /V in CM .

5. Considering that the di ff erential gain of this biopotential ampli fi er is given by the

ratio between the resistor pairs (i.e., G differential 10 M Ω /10 k Ω 1000), calculate

the common-mode rejection ratio, CMRR G differential / G CM , and common-mode

rejection, CMR(dB) 20 log 10 ( G differential / G CM ).

Next, measure the input impedance of the biopotential ampli fi er. You will need an ohm-

meter (e.g., a digital multimeter or VOM) and a 10-M multiturn potentiometer in addi-

tion to the oscilloscope and signal generator. Follow this procedure:

Ω

1. Connect the equipment as depicted in Figure 1.25.
2. Adjust the signal generator to produce a 100-Hz sinusoidal wave with an amplitude
of 1 mV p-p . This signal is measured by channel 2 of the oscilloscope.
3. Set the potentiometer to 0 Ω and measure the ampli fi er's peak-to-peak output volt-

age on channel 1. Record this value.
4. Carefully adjust the 10-M potentiometer until the voltage measured on channel 1

reaches half the value recorded in the preceding step.

Ω